(1) Field of the Invention
The present invention relates to an image forming apparatus which forms images on the recording medium by causing the developer to jump thereto and can be applied to a printer unit in digital copiers and facsimile machines as well as to digital printers, plotters, etc.
(2) Description of the Prior Art
In recent years, image forming means for producing a visual image on recording medium such as paper etc., in response to an image signal have been known and developed. For example, Japanese Patent Application Laid-Open Hei 6 No. 246,956 discloses an image forming apparatus in which control electrodes for controlling the passage of toner are provided in the toner flow control unit, and the areas of the control electrodes are made greater from the upstream side toward the downstream side with respect to the moving direction of the toner support, whereby high quality images are formed at high speed.
Japanese Patent Application Laid-Open Hei 6 No. 344, 588 discloses an image forming apparatus in which the electric field control means is configured so that a lower control voltage than that applied to the control portion located on the downstream side, with respect to the direction in which the toner support is supplying the toner, is applied to the control portion located on the upstream side, so as to enhance the printing density and thereby produce high quality images.
Japanese Patent Application Laid-Open Hei 6 No.91,918 discloses an image forming apparatus in which the voltage applied to each of toner inflow-side electrodes or the voltage application time thereto are varied in accordance with the distance from the toner inflow-side electrode to the toner supplying roller, to thereby reproduce an image with the predetermined density on the recording paper.
Further, Japanese Patent Application Laid-Open Hei 7 No. 89,117 discloses an image forming apparatus in which the control voltage applied to the control portion located on the downstream side with respect to the supplying direction of charged particles is altered in accordance with the controlled state of the control portion located on the upstream side, so as to enhance the printing density and thereby produce high quality images.
In all of the above prior art, charged particles are placed in an electric field so that they will jump by electric force whilst the potential being applied to the control electrode, having a number of passage holes and located in the jump path, is being varied, to thereby make the charged particles adhere to the recording medium, thus forming a visual image on the recording medium, directly. In image forming apparatuses of this type, the jumping of toner is controlled so as to form images. Illustratively, a control means for controlling passage of the charged particles through gates is used to control the electric field formed between the gates and the toner support, and whilst the jumping of toner is controlled, the toner is made to reach to the paper surface by a strong electric field generated by the opposing electrode.
These prior art image forming apparatus, however, suffer from a problem in that the shape and density of formed dots are not uniform resulting in image degradation, for example, producing difficulties in effecting exact reproduction of a halftone dot image. More specifically, in these conventional configurations, when the area on the toner support from which toner jumps is greater than that of the gate formed on the control electrode, there occur cases in which the area on the toner support opposing the gate next to the former gate has an insufficient amount of toner, resultantly causing the problem of white line defects.
In the conventional art, in order to avoid the problem of white line defects, the apparatuses have been configured so that a potential difference is created and/or the size of the electrodes is varied, between the electrodes arranged on the downstream and upstream sides with respect to the position of the toner support. Such configurations might produce negative effects causing downstream dots to be greater in size or in density than upstream dots, resulting in inapt reproduction of suitable halftones. Further, these configurations need a greater number of power sources, and extra diodes and resistors for resistive division, resulting in increase in the number of parts and hence increase in size and cost of the machine as well as degrading the reliability.
There is another known prior art technique in which the voltage permitting the toner to jump (to be referred to as the ON voltage, hereinbelow) to be applied to a gate is increased only when the adjacent gate located directly before it has been used to cause the toner to jump. This prior art configuration, however, needs extra power sources for varying the voltage to be applied, separate voltage switching devices for all individual gates, which means a vast increase in the number of parts and hence increase in size and cost of the machine as well as degradation of the reliability.
Moreover, the image forming apparatuses in accordance with the conventional art may use a cylindrical sleeve as the toner support with a two-dimensional control electrode, in which the distance between each gate and the toner support is not uniform. In such a case, the jumping state of the toner varies depending on the distance. For example, in an area where the distance between the toner support and the control electrode is relatively short, the toner jumps readily because a strong electric field is formed. Inversely, in an area where this distance is greater, an insufficient amount of toner will jump because only a weak electric field can be generated. Further, the greater distance causes the jumping toner to spread and hence make the formed dot greater, resulting in reduction in dot density. Accordingly, if a halftone image, for example, is formed under these conditions, it is impossible to obtain uniform dots, let alone the desired dots, which means lack of any guarantee of suitable image density. A pattern of stripes is liable to arise because high density areas and low density areas are cyclically formed, also resulting in image degradation.
In order to avoid such drawbacks, there is another known method wherein the applied voltage is increased or the voltage application time is made longer in the areas where distances are greater. However, if the voltage application time is made longer, dots form tails so the quality of the dots degrades, and also the printing speed lowers. Further, this configuration needs more power sources and extra parts such as diodes and resistors for varying the voltage, and requires the FETs to be resistive to a higher voltage. Accordingly, it is critically important how to prevent image degradation due to ununiformity of the shape and density of the formed dots when an image forming apparatus of this type is used.